The environmental impact of anthropogenic carbon dioxide emissions, which are currently at about 23.4 Gtonne, is now recognised to be a major risk to mankind.
Carbon Capture and Sequestration (CCS) processes aim to reduce CO2 emissions by capturing CO2 from industrial processes, principally in the power, cement, and steel processes, that burn fossil fuels, and sequestering the CO2 in deep saline aquifers, depleted oil and gas fields, deep coal seams, or deep ocean reservoirs.
There are three approaches to carbon capture for the CCS application—post-combustion capture, pre-combustion capture and oxy-fuel combustion. Pre-combustion capture would be used, for example, in an Integrated Gasification Combined Cycle power plant. However, the initial capital costs of a power plant based on this approach are believed to be very high. Oxy-fuel combustion uses oxygen instead of air, but suffers from the very high cost of separating oxygen from air, and may never be commercially viable. Post-combustion capture is believed to be the most promising CCS process, with the benefit of being more easily integrated into existing power generation systems.
The transport, sequestration and monitoring of CCS are both well established, and their costs are not a hurdle to the adoption of CCS. However, there is currently no established carbon capture process that has been shown to be economically viable for CCS. Only one carbon capture process is commercially used This process, called the MEA process, is currently used by the petroleum industry to separate CO2 from natural gas, where the CO2 has been injected into the reservoir to force out the hydrocarbons. The MEA process separates the natural gas from the CO2, and regenerates the MEA sorbent for a cyclic process. MEA uses amines (and similar materials) as the sorbent, and the reverse process uses steam to release the CO2 to regenerate the amine. The MEA process could operate today as a post combustion process in a power plant at a cost of US $50-70 per tonne of CO2 avoided, well in excess of the target of US $10-20 per tonne of CO2 avoided, as required for the CCS application. MEA cannot be currently used in its present form because it consumes too much energy from the power plant. MEA is a toxic material. Thus there is a world-wide effort to develop new carbon capture technologies that can meet the long term cost target for CCS.
Shimizu et al. (T. Shimizu, T. Hirama, H. Hosoda, K. Kitano: “A twin bed reactor for removal of CO2 from combustion processes”, Trans I Chem E, 77A, 1999) first proposed that a calcination/carbonation cycle be used to capture carbon from flue gases. The paper by Shimizu et al. proposes limestone carbonation at 600° C. for capturing the carbon from the flue gas, and regeneration of CaCO3 above 950° C. by burning fuel with the CaCO3, akin to conventional calcination, with pure oxygen from a separating plant, so as to give pure CO2 and steam as an output. However, this approach is impractical, and limestone calcined above 950° C. will rapidly lose its reactivity due to sintering, as was demonstrated in the work of Shimizu.
Abandades and Alvarez (“Conversion Limits in the Reaction of CO2 with Lime”, Energy & Fuels, 2003, 17, 308-315) presented additional data and reviewed previous work on the Calcium calcination/carbonation cycle. They demonstrated that the fast reaction observed by all researchers was due to the calcination and carbonation of surfaces in micropores of the CaO, which are refilled in carbonation, and a smaller contribution from calcination and carbonation on the larger surfaces. Repeated sintering of the particles during the calcination cycles caused a gradual change in the morphology of the particles with a loss of the micropores, resulting in a loss of the fast component of the carbonation and a degradation of the sorbent.
Garcia at al. (A Garcia, J Carlos and J. Oakey; “Combustion method with integrated CO2 separation by means of carbonation” US Patent publication no. 20060060985) described a process that uses this cycle. They claim a system based on a fluid circulating fluid bed reactor, drawn bed reactor, or cyclone reactor. Their patent discloses that heat transfer from the combustion reactor provides heat to the calciner, and the use of a partial vacuum or steam in the operation of the calciner. They specify a calcination temperature of 900° C. and a carbonation temperature between 600-750° C. They report that the replenishment of the sorbent is 2-5%, so that, on average, the limestone is cycled between 50 and 20 times.
The practical problems with this approach arise firstly from the lifetime of the reactivity of the granules. It is understood that the granules will react with the SOx contaminants in flue gases to produce CaSO3, which is later oxidised to gypsum CaSO4. The injection of limestone granules into hot flue gases to scrub the SOx is an existing technique referred to as “furnace sorbent injection”. In addition, the limestone granules lose reactivity at high temperature in the calcination stage due to sintering. The calciner described by Garcia et at is a fluidized bed to take advantage of the high heat transmission coefficients. Alternatively, a pneumatic transported bed of pipes is described through which the steam is made to pass. The results of A. Abanades and D. Alvarez, Energy and Fuels, 2003, 17, 308-315, shows how the performance of a material that is produced (and later recycled) through 10 minute long calcination steps degrades. The cumulative sintering not only reduces the surface area but closes the pores.
L-S Fan and H. Gupta (US Patent publication no. 20060039853) also described a carbon capture process by limestone using the calcination/carbonation sorption cycle for application in the water gas shift reaction to promote plants hydrogen generation in the water gas shift reaction. They describe the use of a material described in a previous patent (U.S. Pat. No. 5,779,464) as a “super sorbent” as characterised by a high surface area and of 25 m2 gm−1 and a pore volume of 0.05 cm3 gm−1, and a mesoporous pore size distribution in the range of 5-20 nm diameter. Their objective was to make a limestone with a surface that mitigates the effect of “pore clogging”, namely one that has a mesoporous structure, rather than a microporous structure with pores <2 nm.
A critical factor in the assessment of the viability of a CCS system is the energy, capital and operating costs of the processes and the footprint of the capture systems. The energy cost for an efficient regenerable sorbent system is largely determined by the integration of the process into the thermal processes of the power plants or industrial processes and is determined by the recuperation of heat, because the chemical energy of sorption and desorption is recovered. However, any ancillary processes that consume energy such as transfer of granules between reactors would create a penalty. The capital cost translates into the cost of the process, and simple scalable reactor designs are required. The operating costs include the cost of feedstock, and the sorbents used should have a long lifetime and should preferably, be a low cost to manufacture. The operating costs also include the cost of disposal of the spent sorbent, and preferably this should be non-toxic and a waste product that can be profitably consumed. It is understood that a major concern in developing a practical CCS system is the footprint of the reactor systems. Some concepts, when scaled, lead to a CCS system that is as large as the power generator.
A need therefore exists to provide a system and method for calcination/carbonation cycle processing that seeks to address at least one of the above mentioned problems.